Super absorbent polymer

ABSTRACT

The present invention relates to a super absorbent polymer. In the super absorbent polymer according to the present invention, the content of each of the super absorbent polymer particles having different surface shapes among particles constituting the polymer is optimized, and this not only an absorbent capacity and an absorbency under load are excellent but also the rewetting phenomenon can be effectively prevented.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2016/006202, filed on Jun. 10,2016, which claims priority from Korean Patent Application No.10-2015-0084371 filed on Jun. 15, 2015, the disclosures of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a super absorbent polymer having a highabsorption rate and at the same time remarkably improved anti-rewettingeffects.

BACKGROUND OF ART

Super absorbent polymer (SAP) is a synthetic polymer material capable ofabsorbing moisture from about 500 to about 1,000 times its own weight,and each manufacturer has denominated it as different names such as SAM(Super Absorbency Material), AGM (Absorbent Gel Material) or the like.Such super absorbent polymers started to be practically applied insanitary products, and now they are widely used for preparation ofvarious products, for example, hygiene products such as paper diapersfor children or sanitary napkins, water retaining soil products forgardening, water stop materials for the civil engineering andconstruction, sheets for raising seedling, fresh-keeping agents for fooddistribution fields, materials for poultice or the like.

In most cases, these super absorbent polymers have been widely used inthe field of hygienic materials such as diapers or sanitary napkins. Forthese applications, the super absorbent polymer should exhibit amoisture absorbency, it should not release the absorbed water even inthe external pressure, and additionally it should well retain the shapeeven in a state where the volume is expanded (swelled) by absorbingwater, and thereby exhibit excellent liquid permeability.

However, it is known that it is difficult to improve both a centrifugeretention capacity (CRC), which is the physical property showing thebasic absorption capacity and the water retaining capacity of the superabsorbent polymer, and an absorbency under load (AUL), which shows theproperties of well retaining the absorbed moisture even under theexternal pressure. This is because, when the overall crosslinkingdensity of the super absorbent polymer is controlled to be low, thecentrifuge retention capacity can be relatively high, but thecrosslinking structure may be loose, the gel strength may be low andthus the absorbency under load may be lowered. On the contrary, whencontrolling the crosslink density to a high level to improve theabsorbency under load, it becomes difficult for moisture to be absorbedbetween densely crosslinked structures, so that the basic centrifugeretention capacity may be lowered. For the reasons described above,there is a limitation in providing a super absorbent polymer havingimproved centrifuge retention capacity and improved absorbency underload together.

However, recently, as hygiene materials such as a diaper or a sanitarynapkin become thinner, super absorbent polymers are required to havehigher absorption performance. Among these, improving both a centrifugeretention capacity and an absorbency under load which are conflictingphysical properties, improving a liquid permeability, and so on, havebecome an important task.

In addition, pressure can be applied to hygiene materials such asdiapers or sanitary napkins due to the weight of the user. Inparticular, when a super absorbent polymer applied to sanitary materialssuch as diapers or sanitary napkins absorbs liquid and then pressure isapplied due to the weight of the user, a rewetting phenomenon where someliquid absorbed in the super absorbent polymer again leak out can occur.Accordingly, various attempts have been made to improve the absorbencyunder load and the liquid permeability in order to suppress suchrewetting phenomenon. However, concrete methods capable of effectivelysuppressing the rewetting phenomenon have not been suggested.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

For resolving the aforesaid problems of the prior arts, it is an objectof the present invention to provide a super absorbent polymer capable ofeffectively suppressing the rewetting phenomenon while exhibitingexcellent absorption physical properties.

Technical Solution

In order to achieve these objects, the present invention provides asuper absorbent polymer comprising:

-   -   porous particles in which a plurality of pores having an average        pore diameter of 10 μm or more are formed on the surface;    -   secondary granule particles in which primary particles having an        average particle diameter of 10 μm to 100 μm are aggregated; and    -   non-porous particles in which 0 to 3 pores having an average        pore diameter of 5 μm or more are present on the surface,    -   wherein the non-porous particles are contained in an amount of        15% by weight to 75% by weight based on the total weight of the        super absorbent polymer, and    -   the vortex time is 20 seconds to 70 seconds.

According to one embodiment, the super absorbent polymer may include theporous particles in an amount of about 20% by weight to about 60% byweight.

Further, the super absorbent polymer may include the secondary granuleparticles in ah amount of about 10% by weight to about 40% by weight.

Further, according to one embodiment of the present invention, the superabsorbent polymer has, preferably, a centrifuge retention capacity (CRC)for a physiological saline solution of about 28 g/g to about 35 g/g; anabsorbency under load (AUL) at 0.9 psi for a physiological salinesolution of about 14 g/g to about 22 g/g; and a free swell gel bedpermeability (GBP) for a physiological saline solution of about 40 darcyto about 100 darcy.

Meanwhile, the present invention provides a method for preparing a superabsorbent polymer comprising the steps of: carrying out a crosslinkingpolymerization of a monomer mixture comprising a water-solubleethylenicaliy unsaturated monomer having at least partially neutralizedacidic groups, a forming agent, a foam-promoting agent and asilicone-based surfactant in the presence of an internal crosslinkingagent to form a hydrogel polymer; drying, pulverizing and classifyingthe hydrogel polymer to form a base polymer powder; and furthercrosslinking the surface of the base polymer powder in the presence of asurface crosslinking agent to form a surface crosslinked layer.

In the method for preparing a super absorbent polymer, the foaming agentmay include at least one selected from the group consisting of magnesiumcarbonate, calcium carbonate, sodium bicarbonate, sodium carbonate,potassium bicarbonate and potassium carbonate.

Moreover, the silicon-based surfactant may be a polysiloxane containingpolyether side chains.

In addition, the foam-promoting agent may be aluminum salts of inorganicacids and/or aluminum salts of organic acids.

In addition, according to an embodiment of the present invention, thefoaming agent may be present in an amount ranging from about 0.05% toabout 5.0% by weight based on the total weight of the monomer mixture,the foam-promoting agent may be present in an amount ranging from about0.01% to about 3% by weight based on the total weight of the monomermixture, and the silicone-based surfactant may be present in an amountranging from about 0.001% to about 1% by weight based on the totalweight of the monomer mixture.

Advantageous Effects

In the super absorbent polymer according to one embodiment of thepresent invention, the content of each of the super absorbent polymerparticles having different surface shapes among particles constitutingthe polymer is optimized, and thus not only an absorbent capacity and anabsorbency under load are excellent but also the rewetting phenomenoncan be effectively prevented while having a high absorption rate.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 3 are schematic views of an exemplary apparatus for measuringthe gel bed permeability and parts provided in the apparatus.

FIG. 4 is an SEM image of porous particles in which a plurality of poreshaving an average pore diameter of 10 μm or more, which is one componentof the super absorbent polymer according to one aspect of the presentinvention.

FIG. 5 is an SEM image of secondary granule particles, which is onecomponent of the super absorbent polymer according to one aspect of thepresent invention.

FIG. 6 is an SEM image of non-porous particles, which is one componentof the super absorbent polymer according to one aspect of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The super absorbent polymer according to one aspect of the presentinvention comprises porous particles in which a plurality of poreshaving an average pore diameter of 10 μm or more are formed on thesurface; secondary granule particles in which primary particles havingan average particle diameter of 10 μm to 100 μm are aggregated; andnon-porous particles in which 0 to 3 pores having an average porediameter of 5 μm or more are present on the surface, wherein thenon-porous particles are contained in an amount of 15% by weight to 75%by weight based on the total weight of the super absorbent polymer, andthe vortex time is 20 seconds to 70 seconds.

In addition, the method for preparing a super absorbent polymeraccording to one aspect of the present invention comprises the steps of:carrying out a crosslinking polymerization of a monomer mixturecomprising a water-soluble ethylenically unsaturated monomer having atleast partially neutralized acidic groups, a forming agent, afoam-promoting agent and a silicone-based surfactant in the presence ofan internal crosslinking agent to form a hydrogel polymer; drying,pulverizing and classifying the hydrogel polymer to form a base polymerpowder; and further crosslinking the surface of the base polymer powderin the presence of a surface crosslinking agent to form a surfacecrosslinked layer.

Technical terms used in the present specification are only forillustrating specific embodiments, and they are not intended to restrictthe present invention. The singular expressions used herein may includeplural expressions unless the context explicitly indicate otherwise. Itshould be appreciated that the terms such as “including”, “comprising”,or “having” as used herein are intended to embody specific features,numbers, steps, components, and/or combinations thereof, and does notexclude existence or addition of other specific features, numbers,steps, components, and/or combinations thereof.

As the present invention allows for various changes and numerousembodiments, particular embodiments will be illustrated and described indetail below. However, this is not intended to it the present inventionto particular modes of practice, and it is to be appreciated that allchanges, equivalents, and substitutes that do not depart from the spiritand technical scope of the present invention are encompassed in thepresent invention.

As used throughout the present specification, the “porous particle inwhich a plurality: of pores having an average pore diameter of 10 μm ormore are formed on the surface” refers to grains having an irregulargeometric shape in which the particle diameter of each of the particlesis about 150 μm to about 850 μm, the average diameter is about 300 μm toabout 600 μm, a plurality of pores, for examples, about 5 or more, orabout 5 to 20 pores are formed on the surface, and the average diameterof the pores is 10 μm or more, preferably about 10 μm to about 200 μm,and more preferably about 50 μm to about 150 μm.

Also, the term “secondary granule particles” refers to particles inwhich primary particles having an average particle diameter of 10 μm to100 μm are entangled with each other and aggregated in the form ofgranules, the particle diameter of each of the particles is about 150 μmto about 850 μm, the average particle diameter is about 300 μm to about600 μm, the particles are formed into an irregular geometric shape bygranulation, and the surface is formed of a porous structure.

In addition, the “non-porous particles” refers to particles formed intoirregular polygon or irregular geometric particles in which the particlediameter of each of the particles is about 150 μm to about 850 μm, theaverage particle diameter is about 300 μm to about 600 μm, 0 to 3 poreshaving an average pore diameter of 5 μm or more, preferably 0 porehaving an average pore diameter of 5 μm or more, are presented on thesurface thereof, and more preferably no pores are presented on thesurface.

Hereinafter, the present invention will be described in more detail.

The super absorbent polymer according to one aspect of the presentinvention comprises porous particles in which a plurality of poreshaving an average pore diameter of 10 μm or more are formed on thesurface; secondary granule particles in which primary particles havingan average particle diameter of 10 μm to 100 μm are aggregated; andnon-porous particles in which 0 to 3 pores having an average porediameter of 5 μm or more are present on the surface, wherein thenon-porous particles are contained in an amount of 15% by weight to 75%by weight based on the total weight of the super absorbent polymer, andthe vortex time is 20 seconds to 70 seconds.

As the results of experiments by the present inventors, it has beenfound that, when containing all three types of super absorbent polymerparticles having different shapes, particularly containing non-porousparticles having almost no pores on the surface in the above-describedamount, such super absorbent polymer can ensure the surface area, poresize, porosity, etc. of the optimized particles, thereby exhibiting afast absorption rate and a good liquid permeability. In addition, it wasfound that, due to these characteristics, the super absorbent polymercan exhibit a fast absorption rate, excellent absorbency under load andliquid permeability and the like, even in some swollen state, therebyeffectively preventing the rewetting phenomenon where liquid absorbed inthe super absorbent polymer again leaks out by the external pressure.The present invention has been completed on the basis of such finding.

As a result, whatever type of super absorbent polymer is produced so asto include all three types of super absorbent polymer particles havingdifferent shapes as described above, thereby providing a super absorbentpolymer capable of effectively suppressing the rewetting phenomenon.

More specifically, the super absorbent polymer includes porous particlesin which a plurality of pores having an average pore diameter of 10 μmor more are formed on the surface, and the shape and characteristics ofthese particles are as described above.

FIG. 4 is an SEM image of porous particles in which a plurality of poreshaving an average pore diameter of 10 μm or more, which is one componentof the super absorbent polymer according to one aspect of the presentinvention.

Referring to FIG. 4, it can be confirmed that the porous particles,which are one component of the super absorbent polymer according to oneaspect of the present invention, have an irregular geometric shape inwhich the particle diameter is about 150 μm to about 850 μm, a pluralityof pores, about 5 or more pores, are present on the surface, and theaverage pore diameter is 10 μm or more, preferably about 50 μm to about150 μm.

Further, the super absorbent polymer includes secondary granuleparticles in which “primary particles having an average particlediameter of 10 μm to 100 μm” are aggregated, and the shape andcharacteristics of these particles are also as described above.

FIG. 5 is an SEM image of secondary granule particles, which is onecomponent of the super absorbent polymer according to one aspect of thepresent invention.

Referring to FIG. 5, it can be confirmed that the secondary granuleparticles, which are one component of the super absorbent polymeraccording to one aspect of the present invention, are those in whichprimary fine particles having an average particle diameter of about 10μm to about 100 μm are entangled with each other. In addition, it can beconfirmed that the formed secondary granule particles have a particlediameter of about 150 μm to about 850 μm, the granule particles do nothave a uniform geometric shape due to granulation, and the surfacethereof is formed to have a porous structure.

Moreover, the super absorbent polymer includes non-porous particles inwhich 0 to 3 pores having an average pore diameter of 5 μm or more arepresent on the surface, and the shape and characteristics of thenon-porous particles are also as described above.

FIG. 6 is an SEM image of non-porous particles which is one component ofthe super absorbent polymer according to one aspect of the presentinvention.

Referring to FIG. 6, it can be confirmed that the non-porous particles,which are one component of the super absorbent polymer according to oneaspect of the present invention, have irregular polygon or irregulargeometric shapes with particle diameters of about 150 μm to about 850μm, and there are no pores on its surface.

The super absorbent polymer contains the none-porous particles in anamount of about 15% to about 75% by weight. Particularly, when thecontent of the non-porous particles is less than about 15% by weight orexceeds about 75% by weight, there may arise a problem that an excellentabsorption rate in a non-pressurized and a pressurized environmentcannot be secured at the same time. Thereby, there arises a problem thatthe rewetting phenomenon in which the liquid absorbed by thesuperabsorbent resin again leaks out due to the external pressure cannotbe effectively prevented.

According to one embodiment, the super absorbent polymer may include theporous particles in an amount of about 20% to about 60% by weight,preferably about 20% to 50% by weight, or 25% to 40% by weight.

Further, the super absorbent polymer may include the secondary granuleparticles in an amount of about 10% to about 40% by weight, preferablyabout 15% to 35% by weight, or 20% to 30% by weight.

As the super absorbent polymer includes the porous particles and thesecondary granule particles in the above-described range, the superabsorbent polymer can exhibit a high absorption rate and excellent gelstrength even after it is partially swollen.

According to one embodiment of the present invention, the superabsorbent polymer may have a centrifuge retention capacity (CRC) for aphysiological saline solution of about 28 g/g to about 35 g/g,preferably about 28.5 g/g to about 34 g/g or about 29 g/g to about 33g/g. Further, the super absorbent polymer may have an absorbency underload (AUL) at 0.9 psi for a physiological saline solution of about 14g/g to about 22 g/g, preferably about 15 g/g to about 21 g/g or about 18g/g to about 20 g/g. Further, the super absorbent polymer may have afree swell gel bed permeability (GBP) for a physiological salinesolution of about 40 darcy to about 100 darcy, preferably about 45 darcyto about 90 darcy or about 50 darcy to about 80 darcy, and a vortex timeof about 20 seconds to about 70 seconds, preferably about 25 seconds toabout 55 seconds or about 30 seconds to about 50 seconds. In addition,the super absorbent polymer may have a rewetting amount for aphysiological saline solution of less than about 1.5 g/g, preferablyabout 0.01 g/g to about 1.2 g/g, or about 0.1 g/g to about 1.0 g/g.

The super absorbent polymer according to the embodiment showing thesecharacteristics not only has excellent basic absorption characteristicsbut also can exhibit remarkably improved anti-rewetting effects.Thereby, the super absorbent polymer can be applied for various hygieneproducts such as diaper and show very excellent physical properties as awhole.

In particular, when the super absorbent polymer according to theembodiment showing these characteristics contains all three types ofsuper absorbent polymer particles having different shapes and each ofthe particles are contained within a specific range, it, has excellentabsorption rate in a non-pressurized and a pressurized environment atthe same time while having excellent basic physical properties of thesuper absorbent polymer, thereby exhibiting remarkably improvedanti-rewetting effects, high drying efficiency and the like.

On the other hand, the centrifuge retention capacity (CRC) for aphysiological saline solution can be measured according to EDANA(European Disposables and Nonwovens Association) recommended test methodNo. WSP 241.2. More specifically, the centrifuge retention capacity canbe obtained in accordance with the following Calculation Equation 1,after classifying super absorbent polymers to prepare a super absorbentpolymer having a particle diameter of 300 μm to 600 μm, and absorbingthe same in physiological saline solution for 30 minutes:CRC (g/g)={[W ₂ (g)−W ₁ (g)]/W ₀ (g)}−1  (1)in Calculation Equation 1,

W₀ (g) is an initial weight (g) of the super absorbent polymer having aparticle diameter of 300 μm to 600 μm, W₁ (g) is a weight of the devicenot including the super absorbent polymer, measured after dehydratingthe same by using a centrifuge at 250 G for 3 minutes, and W₂ (g) is aweight of the device including a super absorbent polymer, measured aftersoaking and absorbing the super absorbent polymer having the particlediameter of 300 μm to 600 μm in 0.9 wt % physiological saline solutionat room temperature for 30 minutes, and then dehydrating the same byusing a centrifuge at 250 G for 3 minutes.

In addition, the absorbency under load (AUL) at 0.9 psi can be measuredaccording to EDANA recommended test method No. WSP 242.2. Morespecifically, the absorbency under load can be calculated in accordancewith the following Calculation Equation 2, after absorbing the superabsorbent polymer in a physiological saline solution under a load ofabout 0.9 psi over 1 hour:AUL (g/g)=[W ₄ (g)−W ₃ (g)]/W ₀ (g)  (2)in Calculation Equation 2,

W₀ (g) is an initial weight (g) of the super absorbent polymer, W₃ (g)is the total sum of a weight of the super absorbent polymer and a weightof the device capable of providing a load to the super absorbentpolymer, and W₄ (g) is the total sum of a weight of the super absorbentpolymer and a weight of the device capable of providing a load to thesuper absorbent polymer, after absorbing a physiological saline solutionto the super absorbent polymer under a load (0.9 psi) for 1 hour.

W₀ (g) described in Calculation Equations 1 and 2 corresponds to aninitial weight (g) of the super absorbent polymer, before absorbing aphysiological saline solution to the super absorbent polymer, and theymay be the same or different from each other.

The gel bed permeability (GRP) for a physiological saline solution wasmeasured in units of Darcy or cm² according to the following methoddescribed in Korean Patent Application No. 10-2014-7018005. One Darcymeans that it permits a flow of 1 cm³/s of a fluid with viscosity of 1cP under a pressure gradient of 1 atm/cm acting across an area of 1 cm².Gel bed permeability has the same unit as area, and 1 darcy is the sameas 0.98692×10⁻¹² m² or 0.98692×10⁻⁸ cm².

More specifically, as used herein, GBP means a penetration (orpermeability) of a swollen gel layer (or bed) under conditions referredto as 0 psi free swell state (a Gel Bed Permeability (GBP) Under 0 psiSwell Pressure Test), and the GBP can be measured using the apparatusshown in FIGS. 1 to 3.

Referring to FIGS. 1-3, the test apparatus assembly 528 in a device 500for measuring GBP includes a sample container 530 and a plunger 536. Theplunger includes a shaft 538 having a cylinder hole bored down thelongitudinal axis and a head 550 positioned at the bottom of the shaft.The shaft hole 562 has a diameter of about 16 mm. The plunger head isattached to the shaft, for example, by an adhesive. Twelve holes 544 arebored into the radial axis of the shaft, and three holes positioned atevery 90 degrees has a diameter of about 6.4 mm. The shaft 538 ismachined from a LEXAN rod or equivalent material, and has an outerdiameter of about 2.2 cm and an inner diameter of about 16 mm. Theplunger head 550 has seven inner holes 560 and fourteen outer holes 554,all holes having a diameter of about 8.8 mm. Further, a hole of about 16mm is aligned with the shaft. The plunger head 550 is machined from aLEXAN rod or equivalent material and has a height of about 16 mm and adiameter sized such that it fits within the cylinder 534 with minimumwall clearance but still moves freely. The total length of the plungerhead 550 and shaft 538 is about 8.25 cm, but can be machined at the topof the shaft to obtain the desired size of the plunger 536. The plunger536 includes a 100 mesh stainless steel cloth screen 564 that isbiaxially stretched to tautness and attached to the lower end of theplunger 536. The screen is attached to the plunger head 550 using asuitable solvent that causes the screen to be securely adhered to theplunger head 550. Care should be taken to avoid excess solvent movinginto the openings of the screen and reducing the open area for liquidflow area. Acrylic solvent Weld-on 4 from IPS Corporation (having aplace of business in Gardena, Calif., USA) can be used appropriately.The sample container 530 includes a cylinder 534 and a 400 meshstainless steel cloth screen 566 that is biaxially stretched to tautnessand attached to the lower end of the plunger 534. The screen is attachedto the cylinder using a suitable solvent that causes the screen to besecurely adhered to the cylinder. Care should be taken to avoid excesssolvent moving into the openings of the screen and reducing the openarea for liquid flow. Acrylic solvent Weld-on 4 from IPS Corporation(having a place of business in Gardena, Calif., USA) can be usedappropriately. The gel particle sample (swollen super absorbentpolymer), indicated as 568 in FIG. 2, is supported on the screen 566within the cylinder 534 during testing.

Cylinder 534 may be bored from a transparent LEXAN rod or equivalentmaterial, or it may be cut from LEXAN tubing or equivalent material, andhas an inner diameter of about 6 cm (for example, a cross sectional areaof about 28.27 cm²), a wall thickness of about 0.5 cm and a height ofabout 7.95 cm. A step can be formed by machining into the outer diameterof the cylinder 534 such that a region 534 a having an outer diameter of66 mm is present at the bottom 31 mm of the cylinder 534. An O-ring 540which fits the diameter of the region 534 a may be placed on top of thestep.

The annular weight 546 has a counter-bored hole of about 2.2 cm indiameter and 1.3 cm deep so it slides freely onto the shaft 538. Theannular weight also has a thru-bore 548 a of about 16 mm. The annularweight 548 may be made from stainless steel or from other suitablematerial capable of corrosion resistance in 0.9% by weight ofphysiological saline solution (aqueous sodium chloride solution). Thecombined weight of the plunger 536 and the annular weight 548 is equalto about 596 g, which corresponds to a pressure applied to the sample568 of about 0.3 psi or about 20.7 dyne/cm² (2.07 kPa), over a samplearea of about 28.27 cm².

When the test solution flows through the test apparatus during testingof the GBP, the sample container 530 generally rests on a weir 600. Thepurpose of the weir is to divert liquid that overflows the top of thesample container 530, and diverts the overflow liquid to a separatecollection device 601. The weir can be positioned above a scale 602 witha beaker 603 resting on it to collect a physiological saline solutionpassing through the swollen sample 568.

In order to perform the gel bed permeability test under “free swell”conditions, the plunger 536 installed with the weight 548 is placed inan empty sample container 530, and the height from the top of the weight548 to the bottom of the sample container 530 is measured to an accuracyof 0.01 mm using an appropriate gauge. The force to which the thicknessgauge applies during the measurement should be as low as possible,preferably less than about 0.74 N. When using multiple test apparatus,it is important to keep each empty sample container 530, plunger 536 andweight 548 and track of which they are used.

Further, it is preferable that the base on which the sample container530 is placed is flat, and the surface of the weight 548 is parallel tothe bottom surface of the sample container 530. Then, a sample to betested is prepared from the super absorbent polymer for measuring GBP.As an example, a test sample is prepared from a super absorbent polymerhaving a particle diameter of about 300 to about 600 μm, which is passedthrough a US standard 30 mesh screen and retained on a US standard 50mesh screen. About 2.0 g of a sample is placed in a sample container 530and spread out evenly on the bottom of the sample container. Thecontainer containing 2.0 g of sample, without the plunger 536 and theweight 548 therein, is then submerged in the 0.9 wt % physiologicalsaline solution for about 60 minutes and allow the sample to swell underno load condition. At this time, the sample container 530 is placed onthe mesh located in a liquid reservoir so that the sample container 530is raised slightly above the bottom of the liquid reservoir. As themesh, those which do not affect the movement of the physiological salinesolution into the sample container 530 can be used. As such mesh, partnumber 7308 from Eagle Supply and Plastic (having a place of business inAppleton, Wis., USA) can be used. During saturation, the height of thephysiological saline solution can be adjusted such that the surfacewithin the sample container is defined by the sample, rather than thephysiological saline solution.

At the end of this period, the assembly of the plunger 536 and weight548 is placed on the saturated sample 568 in the sample container 530and then the sample container 530, plunger 536, weight 548 and sample568 are removed from the solution. Thereafter, before GBP measurement,the sample container 530, plunger 536, weight 548 and sample 568 areplaced on a flat, large grid non-deformable plate of uniform thicknessfor 30 seconds. The plate will prevent liquid in the sample containerfrom being released onto a flat surface due to surface tension. Theplate has an overall dimension of 7.6 cm×7.6 cm, and each grid has adimension of 1.59 cm long×1.59 cm wide×1.12 cm deep. A suitable platematerial is a parabolic diffuser panel, catalogue number 1624K27,available from McMaster Carr Supply Company (having a place of businessin Chicago, Ill., USA), which can then be cut to the proper dimensions.

Then, if the zero point has not changed from the initial heightmeasurement, the height from the top of the weight 548 to the bottom ofthe sample container 530 is measured again by using the same thicknessgauge as previously used. The height measurement should be made as soonas practicable after the thickness gauge is installed. The heightmeasurement obtained from measuring the empty sample container 530,plunger 536, and weight 548 is subtracted from the height measurementobtained after saturating the sample 568. The resulting value is thethickness, or height “H” of the saturated sample 568. Further, if aplate is contained in the assembly containing the saturated sample 568,the height including the plate should be measured even when measuringthe height of the empty assembly.

The GBP measurement is started by delivering a flow of 0.9%physiological saline solution into the sample container 530 containingthe saturated sample 568, the plunger 536 and the weight 548. The flowrate of physiological saline solution into the container is adjusted tocause physiological saline solution to overflow the top of the cylinder534, thereby resulting in a consistent head pressure equal to the heightof the sample container 530. The physiological saline solution may beadded by any suitable means that is sufficient to ensure a small, butconsistent amount of overflow from the top of the cylinder, such as witha metering pump 604. The overflow liquid is diverted into a separatecollection device 601. The quantity of solution passing through thesample 568 versus time is measured gravimetrically using the scale 602and beaker 603. Data points from the scale 602 are collected everysecond for at least sixty seconds once the overflow has started. Datacollection may be taken manually or with data collection software. Theflow rate (Q) passing through the swollen sample 568 is determined inunits of grams/second (g/s) by a linear least-square fit of fluidpassing through the sample 568 (in grams) versus time (in seconds).

Using the data thus obtained, the gel bed permeability can be confirmedby calculating the GBP (cm²) according to the following CalculationEquation 3.K=[Q×H×μ]/[A×ρ×P]  (3)

in Calculation Equation 3,

N is a gel bed permeability (cm²),

Q is a flow rate (g/sec)

H is a height of swollen sample (cm),

μ is a liquid viscosity (poise) (about one cP for the test solution usedwith this Test),

A is a cross-sectional area for liquid flow (28.27 cm² for the samplecontainer used with this Test),

ρ is a liquid density (g/cm³)(about one g/cm³, for the test solutionused with this Test), and

P is a hydrostatic pressure (dyne/cm²) (normally about 7,797 dynes/cm²).

The hydrostatic pressure is calculated from P=ρ×g×h, where ρ is a liquiddensity (g/cm³), g is a gravitational acceleration (nominally 981cm/sec²), and h is a fluid height (for example, 7.95 cm for the GBP Testdescribed herein).

Meanwhile, the vortex time can be measured in seconds according to themethod described in International Publication. WO 1987/003203. Morespecifically, the vortex time can be calculated by measuring in secondsthe amount of time required for the vortex to disappear after adding 2grams of a super absorbent polymer to 50 mL of physiological salinesolution and then stirring the mixture at 600 rpm.

The super absorbent polymer containing each of the above super absorbentpolymer particles having different surface shapes in a specific amountcan be provided by appropriately adjusting the structure or physicalproperties of various types of super absorbent polymers known in thetechnical field to which the present invention belongs.

More specifically, the super absorbent polymer of one embodimentessentially comprises, as a base polymer powder, a crosslinked polymerobtained by subjecting a water-soluble ethylenically unsaturated monomerto a crosslinking polymerization in the same manner as in the previoussuper absorbent polymer, and may include a surface crosslinked layerformed on these base polymer powders. In addition, the super absorbentpolymer of one embodiment described above includes a structure adjustedso as to contain each of the super absorbent polymer particles havingdifferent surface shapes in a specific content, or an additionalcomponent.

Specifically, a foaming agent capable of generating bubbles duringpolymerization of the base polymer powder, a foam-promoting agent whichpromotes the foam generation and a silicone-based surfactant for stablefoaming can be used to provide a super absorbent polymer containing eachof the super absorbent polymer particles in a specific content.

More specifically, the super absorbent polymer of one embodiment can beprepared by a method comprising the steps of: carrying out acrosslinking polymerization of a monomer mixture including awater-soluble ethylenically unsaturated monomer having at leastpartially neutralized acidic groups, a forming agent, foam-promotingagent and a silicone-based surfactant in the presence of an internalcrosslinking agent to form a hydrogel polymer; drying, pulverizing andclassifying the hydrogel polymer to form a base polymer powder; andfurther crosslinking the surface of the base polymer powder in thepresence of a surface crosslinking agent to form a surface crosslinkedlayer.

In the above-described preparation method, examples of the water-solubleethylenically unsaturated monomer include at least one selected from thegroup consisting of anionic monomers such as acrylic acid, methacrylicacid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid,2-acrylcylethane sulfonic acid, 2-methacryloylethane sulfonic acid,2-(metha)acryloylpropane sulfonic acid, or 2-(meth)acrylamide-2-methylpropane sulfonic acid, or salts thereof; non-ionic hydrophilic monomerssuch as (meth)acrylamide, N-substituted (meth)acrylate,2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethyleneglycol (meth)acrylate, or polyethyleneglycol(meth)acrylate; and amino group-containing unsaturated monomers such as(N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl(meth)acrylamide, or quaternized products thereof. Among these, acrylicacid or a salt thereof, for example, an alkali metal salt such asacrylic acid in which at least a part of the acrylic acid isneutralized, and/or a sodium salt thereof, can be used. By using suchmonomer, it becomes possible to prepare a super absorbent polymer havingmore excellent physical properties. In the case wherein the alkali metalsalt of the acrylic acid is used as the monomer, it is possible to useacrylic acid after neutralizing the same with a basic compound such ascaustic soda (NaOH). In this case, the degree of neutralization of thewater-soluble ethylenically unsaturated monomer can be adjusted to about50% to 95% or about 70% to 85%, and within these ranges, it is possibleto provide a super absorbent polymer having excellent centrifugeretention capacity without fear of precipitation during neutralization.

In the monomer mixture containing the water-soluble ethylenicallyunsaturated monomer, the concentration of the water-solubleethylenically unsaturated monomer may be about 20% to about 60% byweight, or about 40% to about 50% by weight, based on the total weightof the monomer mixture including each raw material and the solvent, andmay be an adequate concentration in consideration of the polymerizationtime and the reaction conditions. However, when the concentration of themonomer is excessively low, the yield of the super absorbent polymer maybe low, and there may be a problem in economic efficiency. In contrast,when the concentration of the monomer is excessively high, it may causeproblems in processes that some of the monomer may be precipitated orthe pulverization efficiency of the prepared hydrogel polymer appearslow in the pulverizing process, and thus the physical properties of thesuper absorbent polymer may decrease.

As the foaming agent used to form a plurality of pores in the basepolymer powder, the form generation can be promoted by thefoam-promoting agent, and a carbonate capable of stable foaming may beused by the silicone-based surfactant.

More specific examines of these carbonates include at least one selectedfrom the group consisting of magnesium carbonate, calcium carbonate,sodium bicarbonate, sodium carbonate, potassium bicarbonate, andpotassium carbonate.

As the foam-promoting agent for promoting the foaming of the foamingagent, an aluminum salt of inorganic acid such as aluminum sulfate oraluminum chloride or an aluminum salt of organic acid such as aluminumlactate, aluminum oxalate, aluminum citrate, aluminum urate and the likemay be used.

If the hydrogel polymer is formed using only the foaming agent and thesilicone-based surfactant without using the foam-promoting agent, or ifthe content of the foaming agent and the foam promoting agent exceedsthe specific range, the optimized porosity and pore structure cannot berealized, and thereby the content of each of the different superabsorbent polymer particles deviates from the above range.

As the silicone-based surfactant for inducing a stable form generationdue to the foaming agent and the foam-promoting agent, a polysiloxanecontaining polyether side chain or the like may be used. Among them, asilicone-based surfactant having a structure in which a polyether sidechain such as poly (ethylene oxide) or poly (propylene oxide) is bondedto a polydimethylsiloxane skeleton can be used. Examples of suchsurfactants include Xiameter® OFX-0190 Fluid (PEG/PPG-18/18Dimethicone), OFX-0193 Fluid (PEG-12 Dimethicone), OFX-5220 Fluid(PEG/PPG-17/18 Dimethicone), OFX-5324 Fluid (PEG-12 Dimethicone), andthe like.

If a hydrogel polymer is formed by using only a foaming agent and afoam-promoting agent without using a silicone-based surfactant or byusing another type of a surfactant other than a silicone basedsurfactant, the super absorbent polymer is formed into an excessivelyporous structure, the centrifuge retention capacity and the absorbencyunder load are lowered, and the bulk density is low, which makes itdifficult to handle at the time of classification or the like.Accordingly, when the silicone-based surfactant is not used, the averageparticle diameter of the gel partially swollen under the above-describedconditions according to one embodiment is deviated from the above range.

In the monomer mixture containing the water-soluble ethylenicallyunsaturated monomer or the like, the concentration of the foaming agentmay be about 0.05% to about 5.0% by weight, preferably about 0.1% toabout 3.0% by weight or about 0.15% to about 3% by weight, based on thetotal weight of the monomer mixture. The concentration of thefoam-promoting agent may be about 0.01% to about 3% by weight or about0.15% to about 2% by weight, based on the total weight of monomermixture. The concentration of the silicone-based surfactant may be about0.001% to about 1% by weight or about 0.01% to about 0.5% by weight,based on the total weight of monomer mixture.

When a foaming agent, a foam-promoting agent, and a silicone-basedsurfactant are used in these ranges, the pore size and the porosity etc.of the super absorbent polymer can be optimized and the absorptionsurface area can be remarkably improved, resulting in an improvement inthe absorption rate and anti-rewetting effect.

Meanwhile, in order to allow a large amount of forms or bubblesgenerated by using the foaming agent, the foam-promoting agent and thesilicone-based surfactant to be contained in the hydrogel polymer, andto stably maintain a plurality of pores contained in the hydrogelpolymer even in a subsequent process, the hydroxyl group-containingcompound may be used in the step of forming the hydrogel polymer.

More specifically, when the hydroxy group-containing compound is used inthe step of forming the hydrogel polymer, the viscosity of the polymersolution can be improved during the crosslinking polymerization of themonomer mixture containing the water-soluble ethylenically unsaturatedmonomer or the like, thereby shortening the gelation time. This makes itto effectively prevent a large amount of forms or bubbles generated bythe foaming agent or the like from dropping off from the polymerizationliquid, so that a large amount of foams or bubbles are contained in thehydrogen polymer. In addition, the hydroxy group-containing compound canbe included in the finally produced super absorbent polymer, thusimproving the wettability of the super absorbent polymer. Thereby, thenon-pressurized absorption rate and the pressurized absorption rate ofthe super absorbent polymer can be further increased.

The such hydroxy group-containing compounds that can be used herein maybe compounds such as polyvinyl alcohol, or polyalkylene glycols such aspolyethylene glycol. The concentration of such hydroxy group-containingcompound can be used in an amount of about 0.1% to 1% by weight based onthe total weight of the monomer mixture. The absorption area andwettability of the super absorbent polymer can be effectively increasedwithin the range of these contents.

As the internal crosslinking agent for introducing the basiccrosslinking structure into the base polymer powder, any internalcrosslinking agent having a crosslinkable functional groupconventionally used for producing a super absorbent polymer can be usedwithout any limitation. However, in order to improve the physicalproperties of the super absorbent polymer by introducing an appropriatecrosslinking structure into the base polymer powder, polyfunctionalacrylate compound having a plurality of ethylene oxide groups may beused as an internal crosslinking agent. More specific examples of suchinternal crosslinking agent include at least one selected from the groupconsisting of polyethylene glycol diacrylate (PEGDA), glycerindiacrylate, glycerin triacrylate, unmodified or ethoxylatedtrimethylolpropane triacrylate (TMPTA), hexanediol diacrylate, andtriethylene glycol diacrylate. Such internal crosslinking agent may beincluded at a concentration of about 0.01% to about 0.5% by weight basedon the monomer mixture to crosslink the polymerized polymer.

In addition, the monomer mixture may further include a polymerizationinitiator generally used for producing a super absorbent polymer.

Specifically, the polymerization initiator that can be used hereinincludes a thermal polymerization initiator or a photopolymerizationinitiator by UV irradiation, depending on the polymerization method.However, even in the case of using the photopolymerization method,because a certain amount of heat is generated by the ultravioletirradiation or the like and a certain degree of heat is generatedaccording to the progress of the exothermic polymerization reaction, athermal polymerization initiator may be additionally included.

The photopolymerization initiator can be used without any limitation aslong as it is a compound capable of forming a radical by a light such asan UV ray.

The photopolymerization initiator may include, for example, at least oneinitiator selected from the group consisting of a benzoin ether, adialkyl acetophenone, a hydroxyl alkylketone, a phenyl glyoxylate, abenzyl dimethyl ketal, an acetyl phosphine, and an α-aminoketone.Meanwhile, specific examples of the acyl phosphine may include normallucirin TPO, namely, 2,4,6-trimethyl-benzoyl-trimethyl phosphine oxide.More various photopolymerization initiators are well disclosed in “UVCoatings: Basics, Recent Developments and New Application” written byReinhold Schwalm, (Elsevier, 2007), p 115, however thephotopolymerization initiator is not limited to the above-describedexamples.

The photopolymerization initiator may be included in the concentrationof about 0.01% to about 1.0% by weight based on the monomer mixture.When the concentration of the photopolymerization initiator isexcessively low, the polymerization rate may become slow, and when theconcentration of the photopolymerization initiator is excessively high,the molecular weight of the super absorbent polymer becomes small andits physical properties may become uneven.

And, as the thermal polymerization initiator, one or more initiatorsselected from the group consisting of a persulfate-based initiator, anazo-based initiator, hydrogen peroxide, and ascorbic acid may be used.Specific examples of the persulfate-based initiator may include sodiumpersulfate (Na₂S₂O₈), potassium persulfate (K₂S₂O₈), ammonium persulfate((NH₄)₂S₂O₈), and the like; and examples of the azo-based initiator mayinclude 2,2-azobis-(2-amidinopropane)dihydrochloride,2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride,2-(carbamoylazo)isobutyionitrile, 2,2-azobis-[2-(2-imidazolin-2-yl)propane]dihydrochloride, 4,4-azobis-(4-cyanovaleric acid) and the like.More various thermal polymerization initiators are well disclosed in“Principle of Polymerization” written by Odian, (Wiley, 1981), p 203,however the thermal polymerization initiator is not limited to theabove-described examples.

The thermal polymerization initiator can be included in theconcentration of about 0.001% to about 0.5% by weight based on themonomer mixture. When the concentration of the thermal polymerizationinitiator is excessively low, the additional thermal polymerizationhardly occurs and thus effects due to the addition of the thermalpolymerization initiator may be insignificant, and when theconcentration of the thermal polymerization initiator is excessivelyhigh, the molecular weight of the super absorbent polymer becomes smalland the physical properties may become uneven.

In addition, the monomer mixture may further include additives such as athickener, a plasticizer, a preservation stabilizer, an antioxidant,etc., if necessary.

The raw materials such as the water-soluble ethylenically unsaturatedmonomer, the forming agent, the foam-promoting agent, the silicone-basedsurfactant, the photopolymerization initiator, the thermalpolymerization initiator, the internal crosslinking agent, and theadditives may be prepared in the form of the monomer mixture solutionwhich is dissolved in a solvent.

The solvent that can be used herein is not limited as long as it candissolve the above-described components. For example, one or moresolvents selected from the group consisting of water, ethanol,ethyleneglycol, diethyleneglycol, triethyleneglycol, 1,4-butanediol,propyleneglycol, ethyleneglycol monobutylether, propyleneglycolmonomethylether, propyleneglycol monomethylether acetate,methylethylketone, acetone, methylamylketone, cyclohexanone,cyclopentanone, diethyleneglycol monomethylether, diethyleneglycolethylether, toluene, xylene, butylolactone, carbitol, methylcellosolveacetate, and N,N-dimethyl acetamide, and so on may be used alone or incombination with each other.

The solvent may be included in a residual amount of excluding theabove-described components from the total weight of the monomer mixture.

Meanwhile, the method for preparing a hydrogel polymer by the thermalpolymerization or photopolymerization of the monomer mixture may betypically carried out in a reactor like a kneader equipped withagitating spindles to facilitate the foam generation.

The hydrogel polymer, which is discharged from the outlet of the reactorby supplying a polymerization energy source such as heat or light to areactor such as a kneader having an agitating shaft as described above,may have a size of centimeters or millimeters, depending on the type ofthe agitating spindles equipped in the reactor. Specifically, the sizeof the hydrogel polymer obtained may vary depending on the concentrationof the monomer mixture injected thereto, the injection rate or the like,and the hydrogel polymer having a weight average particle diameter of 2mm to 50 mm can be generally obtained.

The hydrogel polymer obtained by such method may have typically amoisture content of about 40% to about 80% by weight. Meanwhile, theterm “moisture content” as used herein refers to the content of moisturein the total weight of the hydrogel polymer, which is obtained bysubtracting the weight of the dried polymer from the weight of thehydrogel polymer. Specifically, it is defined as a value calculated bymeasuring the weight loss according to evaporation of water in thepolymer during the drying process by increasing the temperature of thepolymer through infrared heating. In this case, the moisture content ismeasured under the drying conditions where the temperature is increasedfrom room temperature to 180° C. and then the temperature is maintainedat 180° C., and the total drying time is set to 20 minutes, including 5minutes for the temperature rising step.

After the monomer is subjected to crosslinking polymerization, the basepolymer powder can be obtained through drying, pulverization,classification, etc., and through a process such as pulverization andclassification, the base polymer powder, and the super absorbent resinobtained therefrom are suitably manufactured and provided so as to havea particle diameter of about 150 μm to 850 μm. More specifically, atleast about 95% by weight or more of the base polymer powder and t superabsorbent resin obtained therefrom has a particle diameter of about 150μm to 850 μm and the fine particles having a particle diameter of lessthan about 150 μm can be less than about 3% by weight.

As the particle diameter distribution of the base polymer powder and thesuper absorbent polymer is adjusted to the preferable range, the superabsorbent polymer finally produced can exhibit excellent absorbentproperties.

On the other hand, the method of drying, pulverization andclassification will be described in more detail below.

First, when drying the hydrogel polymer, a coarsely pulverizing step maybe further carried out before drying in order to increase the efficiencyof the drying step, if necessary.

A pulverizing machine used herein may include, but its configuration isnot limited to, for example, any one selected from the group consistingof a vertical pulverizer, a turbo cutter, a turbo grinder, a rotarycutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, achopper, and a disc cutter. However, it is not limited to theabove-described examples.

In this case, the coarsely pulverizing step may be carried out so thatthe particle diameter of the hydrogel polymer becomes about 2 mm toabout 10 mm.

Pulverizing the hydrogel polymer into a particle diameter of less than 2mm is technically not easy due to its high moisture content, andagglomeration may occur between the pulverized particles. Meanwhile, ifthe polymer is pulverized into a particle diameter of greater than 10mm, the effect of increasing the efficiency in the subsequent dryingstep may be insignificant.

The hydrogel polymer coarsely pulverized as above or the hydrogelpolymer immediately after polymerization without the coarselypulverizing step is subjected to a drying step. In this case, the dryingtemperature of the drying step may be about 150° C. to about 250° C.

When the drying temperature is less than 150° C., it is likely that thedrying time becomes too long or the physical properties of the superabsorbent polymer finally formed is deteriorated, and when the dryingtemperature is higher than 250° C., only the surface of the polymer isdried, and thus it is likely that fine powder is generated during thesubsequent pulverizing step and the physical properties of the superabsorbent polymer finally formed is deteriorated.

Meanwhile, the drying time may be about 20 to about 90 minutes, inconsideration of the process efficiency and the like, but it is notlimited thereto.

In the drying step, the drying method may also be selected and usedwithout any limitation if it is a method generally used for drying thehydrogel polymer. Specifically, the drying step may be carried out by amethod such as hot air supply, infrared irradiation, microwaveirradiation or ultraviolet irradiation. When the drying step as above isfinished, the moisture content of the polymer may be about 0.1% to about10% by weight.

Subsequently, the dried polymer obtained through the drying step issubjected to a pulverization step.

The polymer powder obtained through the pulverizing step may have aparticle diameter of about 150 μm to about 850 μm. Specific examples ofa pulverizing device that can be used to achieve the above particlediameter may include a pin mill, a hammer mill, a screw mill, a rollmill, a disc mill, a jog mill or the like, but the present invention isnot limited thereto.

Also, in order to control the physical properties of the super absorbentpolymer powder finally commercialized after the pulverization step, aseparate step of classifying the polymer powder obtained after thepulverization depending on the particle diameter may be undergone.Preferably, a polymer having a particle diameter of about 150 μm toabout 850 μm is classified and only the polymer powder having such aparticle diameter is subjected to the surface crosslinking reaction andfinally commercialized. Since the particle diameter distribution of thebase polymer powder obtained through such a process has already beendescribed above, a further detailed description thereof will be omitted.

On the other hand, after the step of forming the base polymer powderdescribed above, the surface cross-linked layer can be formed by furthercrosslinking the surface of the base polymer powder in the presence ofthe surface crosslinking agent, whereby a super absorbent polymer can beproduced.

The surface crosslinked layer can be formed by using a surfacecrosslinking agent conventionally used in the production of a superabsorbent polymer. As the surface crosslinking agent, any one known inthe technical field to which the present invention belongs can be usedwithout any limitation. More specific examples thereof include at leastone selected from the group consisting of ethylene glycol, propyleneglycol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol,2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol,2-methyl-2,4-pentanediol, tripropylene glycol, glycerol, ethylenecarbonate and propylene carbonate. Such surface crosslinking agent maybe used in an amount of about 0.01 to 3% by weight based on the totalweight of the base polymer powder.

In the surface crosslinking step, the surface crosslinking reaction canbe carried out by further adding one or more inorganic substancesselected from the group consisting of silica, clay, alumina,silica-alumina composite material, titania, zinc oxide and aluminumsulfate. The inorganic substance can be used in the form of powder orliquid, and in particular, it can be used as alumina powder,silica-alumina powder, titania powder or nanosilica solution. Inaddition, the inorganic material can be used in an amount of about 0.05%to about 2% by weight based on the total weight of the base polymerpowder.

Moreover, in the surface crosslinking step, as the surface crosslinkingproceeds by adding a polyvalent metal cation in place of the inorganicsubstance or together with the inorganic substance, the surfacecrosslinking structure of the super absorbent polymer can be furtheroptimized. This is presumably because such a metal cation can furtherreduce the crosslinking distance by forming a chelate with the carboxylgroup (COOH) of the super absorbent polymer.

There is also no limitation on the method of adding the surfacecrosslinking agent, and optionally an inorganic substance and/or an apolyvalent metal cation to the base polymer powder. For example, amethod of adding surface crosslinking agent and a base polymer powder toa reaction tank and mixing them, or a method of spraying a surfacecross-linking agent or the like to the base polymer powder, or a methodof adding a base polymer powder and a surface crosslinking agent to acontinuously operated mixer and mixing them, or the like, may be used.

When the surface crosslinking agent is added, water and methanol can beadditionally mixed and added. When water and methanol are added, thereis an advantage that the surface crosslinking agent can be uniformlydispersed in the base polymer powder. At this time, the amount of waterand methanol added can be appropriately adjusted in order to induce amore uniform dispersion of the crosslinking agent, prevent theaggregation phenomenon of the polymer powders, and further optimize thedepth of penetration of the surface crosslinking agent to the polymerpowders.

The surface crosslinking reaction may be performed by heating the basepolymer powder to which the surface crosslinking agent is added at atemperature of about 100° C. or more for about 20 minutes or more. Inparticular, in order to produce a super absorbent polymer that moresuitably fulfills physical properties according to one embodiment, theconditions of the surface crosslinking step can adjust the maximumreaction temperature to about 100° C. to 250° C. The retention time atthe maximum reaction temperature can be adjusted under the conditions ofabout 20 minutes or more, or about 20 minutes or more and 1 hour orless. In addition, at a temperature at the beginning of the firstreaction, for example, at a temperature of about 100° C. or more, thetemperature raising time until reaching the maximum reaction temperaturecan be controlled to be about 10 minutes or more, or about 10 minutes ormore and 1 hour or less.

A temperature raising means for surface crosslinking is not particularlylimited. Heating medium may be supplied or heat source may be directlysupplied and heated. The kind of heating medium that can be used hereinmay include steam, hot wind, temperature-raised fluid such as hot oil,and the like, but is not limited thereto, and the temperature of thesupplied heating medium may be appropriately selected in considerationof the means of heating medium, temperature-raising speed, and targettemperature. Meanwhile, the directly supplied heat source may includeelectric heating, gas heating, but is not limited to the above examples.

The super absorbent polymer obtained according to the above-describedpreparation method can be partially swollen under the above-mentionedconditions to easily form a gel having a uniform particle diameterdistribution, so that the partially swollen gel can exhibit an averageparticle diameter in the above-mentioned range. That is, the superabsorbent polymer obtained according to the above-described preparationmethod is optimized pore size and porosity due to the use of a foamingagent, a foam-promoting agent and a silicone-based surfactant, and thusit can exhibit excellent absorption rate and high gel strength evenunder pressurized and non-pressurized conditions, thereby effectivelypreventing the rewetting phenomenon.

Hereinafter, the action and effects of the present invention will bedescribed in detail by way of specific Examples. However, these Examplesare given for illustrative purposes only, and the scope of the inventionis not intended to be limited thereto.

Example 1: Preparation of Super Absorbent Polymer

A solution (solution A) in which 11 g of 0.5% IRGACUPE 819 initiator(110 ppm based on the monomer mixture) diluted with acrylic acid and 26g of 5% polyethylene glycol diacrylate (PEGDA, molecular weight 400)diluted with acrylic acid were mixed was reared.

Then, a solution (solution B) of 5% trimethylolpropane triacrylatecontaining 9 mol % of ethylene oxide (Ethoxylated-TMPTA, IMP (EO) 9TA,M-3190 manufactured by Miwon Specialty Chemical Co., Ltd.) diluted withacrylic acid was prepared.

37 g of the solution A was injected into a 2 L glass reactor surroundedby a jacket through which a heat medium pre-cooled at 25° C. wascirculated, to which 14 g of the solution B was injected. 1% OFX-0193(XIAMETER®) diluted with acrylic acid was injected as the silicone-basedsurfactant to the glass reactor and mixed. Then, 800 g of 24% causticsoda solution (solution C) was slowly added dropwise to the glassreactor and mixed. After the temperature of the mixture increased to 72°C. or higher by neutralization heat upon dropwise addition of thesolution C, the mixed solution was left until it was cooled. The degreeof neutralization of acrylic, acid in the mixed solution thus obtainedwas about 70 mol %.

Meanwhile, 5% sodium bicarbonate solution (solution D) diluted withwater and 28 g of 4% sodium persulfate solution diluted with water weredissolved to prepare a solution (solution E-1).

Then, when the temperature of the mixed solution was cooled to about 45°C., 14 g of the solution D previously prepared was injected into themixed solution and mixed, and at the same time, the solution E-1 wasinjected.

Then, the above prepared solution was poured in a Vat-type tray (15 cmin width×15 cm in length) installed in a square polymerizer which had alight irradiation device installed at the top and was preheated to 80°C. Subsequently, the mixed solution was irradiated with light. It wasconfirmed that a gel was formed on the surface after about 20 secondsfrom light irradiation, and it was confirmed that polymerizationreaction occurred simultaneously with foaming after about 30 secondsfrom light irradiation. Subsequently, the reaction was allowed foradditional 2 minutes, and the polymerized sheet was taken out and cutinto a size of 3 cm×3 cm. Then, the cut sheet was subjected to a crumbthrough a chopping process using a meat chopper to prepare crumbs.

The crumbs were then dried in an oven capable of shifting airflow up anddown. The crumbs were uniformly dried by flowing hot air at 180° C. fromthe bottom to the top for 15 minutes and from the top to the bottom for15 minutes, so that the dried product had a water content of 2% or less.

The dried product was pulverized using a pulverizer and classified toobtain a base polymer having a particle diameter of 150 to 850 μm. Thebase polymer thus prepared had a centrifuge retention capacity of 36.5g/g and a water-soluble component content of 14.2% by weight. Thecentrifuge retention capacity was measured according to EDANArecommended test method No. WSP 241.2 and the water-soluble componentcontent was measured according to EDANA recommended test method No. WSP270.2

Thereafter, 100 g of the base polymer was mixed with a crosslinkingagent solution obtained by mixing 3 g of water, 3 g of methanol, 0.4 gof ethylene carbonate, and 0.5 g of Aerosil 200 (EVONIK), and thensurface crosslinking reaction was carried out at 190° C. for 30 minutes.The resultant was pulverized and sieved to obtain a surface-crosslinkedsuper absorbent polymer having a particle diameter of 150 to 850 μm.

Example 2: Preparation of Super Absorbent Polymer

A base polymer was prepared in the same manner as in Example 1, exceptthat the solution D was used in an amount of 15 g instead of 34 g. Thebase polymer thus prepared had a centrifuge retention capacity of 35.2g/g and a water-soluble component content of 13.9% by weight. Thesurface-crosslinked super absorbent polymer having a particle diameterof 150 to 850 μm was obtained in the same manner as in Example 1, byusing the prepared base polymer.

Example 3: Preparation of Super Absorbent Polymer

0.1 g of Aerosil 200 (Aerosil 200, EVONIK) was added to 100 g of thesurface-crosslinked super absorbent polymer prepared in Example 1 andthe mixture was dry-mixed to obtain a super absorbent polymer.

Example 4: Preparation of Super Absorbent Polymer

A base polymer was prepared in the same manner as in Example 1, exceptthat the solution (E-2 solution) in which 0.8 g of aluminum sulfate wasdissolved in 28 g of 4% sodium persulfate solution diluted with waterwas used instead of the solution E-1. The base polymer thus prepared hada centrifuge retention capacity of 37.4 g/g and a water-solublecomponent content of 15.1% by weight. The surface-crosslinked superabsorbent polymer having a particle diameter of 150 to 850 μm wasobtained in the same manner as in Example 1, by using the prepared basepolymer.

Example 5: Preparation of Super Absorbent Polymer

A solution (solution A) in which 11 g of 0.5% IRGACURE 819 initiator.(110 ppm based on the monomer mixture) diluted with acrylic acid and 26g of 5% polyethylene glycol diacrylate (PEGDA, molecular weight: 400)diluted with acrylic acid were mixed was prepared.

Then, a solution (solution B) of 5% trimethylolpropane triacrylatecontaining 9 mol % of ethylene oxide (Ethoxylated-TMPTA, TMP (EO) 9TA,M-3190 manufactured by Miwon Specialty Chemical Co., Ltd.) diluted withacrylic acid was prepared.

37 g of the solution A was injected into a 2 L glass reactor surroundedby a jacket through which a heat medium pre-cooled at 25° C. wascirculated, to which 14 g of the solution B was injected. Then, 800 g of24% caustic soda solution (solution C) was slowly added dropwise to theglass reactor and mixed. After the temperature of the mixture wasincreased to 72° C. or higher by neutralization heat upon dropwiseaddition of the solution C, the mixed solution was left until it wascooled. The degree of neutralization of acrylic acid in the mixedsolution thus obtained was about 70 mol %.

Meanwhile, 5% sodium bicarbonate solution (solution)) diluted withwater, a solution (solution E-2) in which 1.6 g of aluminum sulfate wasdissolved in 28 of 5% sodium bicarbonate solution diluted with water,and 1% OFX-0193 (XIAMETER®) solution (solution F) diluted with waterwere prepared.

Then, when the temperature of the mixed solution was cooled to about 45°C., a mixture of the solution. D and the solution F previously preparedwas injected to the mixed solution and mixed, and at the same time, thesolution E-2 was injected.

Then, the above prepared solution was poured in a Vat-type tray (15 cmin width×15 cm in length) installed in a square polymerizer which had alight irradiation device installed at the top and was preheated to 80°C. Subsequently, the mixed solution was irradiated with light. It wasconfirmed that a gel was formed on the surface after about 20 secondsfrom light irradiation, and it was confirmed that polymerizationreaction occurred simultaneously with foaming after about 30 secondsfrom light irradiation. Subsequently, the reaction was allowed foradditional 2 minutes, and the polymerized sheet was taken out and cutinto a size of 3 cm×3 cm. Then, the cut sheet was subjected to crumbthrough a chopping process using a meat chopper to prepare crumbs.

The crumbs were then dried in an oven capable of shifting airflow up anddown. The crumbs were uniformly dried by flowing hot air at 180° C. fromthe bottom to the top for 15 minutes and from the top to the bottom for15 minutes, so that the dried product had a water content of 2% or less.

The dried product was pulverized using a pulverizer and classified toobtain a base polymer having a particle diameter of 150 to 850 μm. Thebase polymer thus prepared had a centrifuge retention capacity of 35.8g/g and a water-soluble component content of 13.7% by weight.

Thereafter, 100 g of the base polymer was mixed with a crosslinkingagent solution obtained by mixing 3 g of water, 3 g of methanol, 0.4 gof ethylene carbonate, and 0.5 g of Aerosil 200 (EVONIK), and thensurface crosslinking reaction was carried out at 190° C. for 30 minutes.The resultant was pulverized and sieved to obtain a surface-crosslinkedsuper absorbent polymer having a particle diameter of 150 to 850 μm.

Comparative Example 1: Preparation of Super Absorbent Polymer

A base polymer was prepared in the same manner as in Example 5, exceptthat 28 g of 4% sodium bicarbonate solution (solution E-0) diluted withwater was injected instead of the mixture of the solution D and thesolution F and the solution E-2. The base polymer thus prepared had acentrifuge retention capacity of 39.3 g/g and a water-soluble componentcontent of 19.3% by weight. The surface-crosslinked super absorbentpolymer having a particle diameter of 150 to 850 μm was obtained in thesame manner as in Example 5, by using the prepared base polymer.

Experimental Example: Evaluation of Super Absorbent Polymer

The properties of the super absorbent polymers prepared in Examples 1 to5 and Comparative Example 1 were evaluated by the following methods, andthe results are shown in Table 1 below.

(1) Average Particle Diameter of Super Absorbent Polymer

The average particle diameter of each of the super absorbent polymersprepared in Examples 1 to 5 and Comparative Example 1 was measuredaccording to EDANA WSP 220.2 (European Disposables and NonwovensAssociation, EDANA).

(2) Confirmation of the Content of Each Particle According to Shape

Powder samples of the super absorbent polymers prepared in Examples 1 to5 and Comparative Example 1 were well placed a base plate and then thesurface shape was observed with a scanning electron microscope (2.0 kV).

An image was obtained with a width of 3 mm and a length of 2 mm of aphotograph enlarged by a scanning electron microscope at a magnificationof 50 times, and the particles contained in the image were distinguishedinto porous particles, secondary granule particles, and non-porousparticles. The content (% by weight) was calculated by using theparticle diameter of the particles observed and the average densityvalue of each particle.

(Density of porous particles: 0.55 g/mm³, density of secondary granuleparticles: 0.55 g/mm³, density of non-porous particles: 0.65 g/mm³).

(3) Centrifuge Retention Capacity (CRC)

The centrifuge retention capacity (CRC) for a physiological salinesolution was measured for each super absorbent polymer prepared inExamples 1 to 5 and Comparative Example 1 according to EDANA recommendedtest method No. WSP 241.2.

Specifically, a super absorbent polymer having a particle diameter of300 to 600 μm which was passed through a U.S. standard 30 mesh screenand retained on a U.S. standard 50 mesh screen was prepared from a superabsorbent polymer for evaluating the centrifuge retention capacity.

Then, the super absorbent polymer W₀ (g, about 0.2 g) having a particlediameter of 300 to 600 μm was uniformly placed into a nonwovenfabric-made bag, followed by sealing. Then, the bag was immersed in 0.9%by weight of physiological saline solution at room temperature. After 30minutes, the bag was dehydrated at 250 G for 3 minutes with acentrifuge, and the weight W₂ (g) of the bag was then measured.Meanwhile, after carrying: out the same procedure using an empty bag notcontaining a super absorbent polymer, the resultant weight W₁ (g) wasmeasured.

Using the respective weights thus obtained, a centrifuge retentioncapacity was confirmed according to the following Calculation Equation1:CRC (g/g)={[W ₂ (g)−W ₁ (g)]/W ₉ (g)}−1  (1)

in Calculation Equation 1,

W₀ (g) is an initial weight (g) of the super absorbent polymer having aparticle diameter of 300 μm to 600 μm,

W₁ (g) is a weight of the device not including the super absorbentpolymer, measured after dehydrating the same by using a centrifuge at250 G for 3 minutes, and

W₂ (g) is a weight of the device including a super absorbent polymer,measured after soaking and absorbing the super absorbent polymer havinga particle diameter of 300 μm to 600 μm in 0.9% by weight ofphysiological saline solution at room temperature for 30 minutes, andthen dehydrating the same by using a centrifuge at 250 G for 3 minutes.

(4) Absorbency Under Load (AUL)

The absorbency under load (AUL) at 0.9 psi for a physiological salinesolution was measured for each super absorbent polymer prepared inExamples 1 to 5 and Comparative Example 1 according to EDANA recommendedtest method No. WSP 242.2.

Specifically, a 400 mesh stainless steel net was installed in the bottomof a plastic cylinder having an inner diameter of 25 mm. W₀ (g, 0.16 g)of a super absorbent polymer for measuring the absorbency under loadwere uniformly scattered on the screen under conditions of roomtemperature and relative humidity of 50%. Then, a piston which couldprovide a load of 6.3 kPa (0.9 psi) uniformly was put thereon. At thistime, the piston used was designed so that the outer diameter wasslightly smaller that 25 mm and thus it could move freely up and downwithout any gap with the inner wall of the cylinder. Then, the weight W₃(g) of the device prepared in this way was measured.

After putting a glass filter having a diameter of 90 mm and a thicknessof 5 mm in a Petri dish having the diameter of 150 mm, 0.90% by weightof a physiological saline solution was poured in the dish. At this time,the physiological saline solution was poured until the surface levelbecame equal to the upper surface of the glass filter. Then, a sheet offilter paper having a diameter of 90 mm was put on the glass filter.

Subsequently, the prepared device was placed on the filter paper so thatthe super absorbent polymer in the device was swelled by a physiologicalsaline solution under load. After one hour, the weight W₄ (g) of thedevice containing the swollen super absorbent polymer was measured.

Using the weight thus measured, the absorbency under load was calculatedaccording to the following Calculation Equation 2.AUL (g/g)=[W ₄ (g)−W ₃ (g)]/W ₀ (g)  (2)

in Calculation Equation 2,

W₀ (g) is an initial weight (g) of the super absorbent polymer, W₃ (g)is the total sum of a weight of the super absorbent polymer and a weightof the device capable of providing a load to the super absorbentpolymer, and W₄ (g) is the total sum of a weight of the super absorbentpolymer and a weight of the device capable of providing a load to thesuper absorbent polymer, after absorbing a physiological saline solutionto the super absorbent polymer under a load (0.9 psi) for 1 hour.

(5) Gel Bed Permeability (GBP)

The gel bed permeability (GBP) for a physiological saline solution wasmeasured for each super absorbent polymer prepared in Examples 1 to 5and Comparative Example 1 according to the following method described inKorean Patent Application No. 10-2014-7018005.

Specifically, the apparatus shown in FIGS. 1 to 3 was used to measurethe free swell GBP. First, the plunger 536 installed with the weight 548was placed in an empty sample container 530, and the height from the topof the weight 548 to the bottom of the sample container 530 was measuredto an accuracy of 0.01 mm using an appropriate gauge. The force to whichthe thickness gauge applied during the measurement was adjusted to lessthan about 0.74 N.

Meanwhile, a super absorbent polymer having a particle diameter of about300 to about 600 μm was obtained by selectively classifying a superabsorbent polymer which was passed through a US standard 30 mesh screenand retained on a US standard 50 mesh screen.

About 2.0 g of the super absorbent polymer classified in this way wasplaced in the sample container 530 and spread out evenly on the bottomof the sample container. Then, the container not containing the plunger536 and the weight 548 therein, was submerged in 0.9 wt % physiologicalsaline solution for about 60 minutes and allowed the super absorbentpolymer to swell under no load condition. At this time, the samplecontainer 530 was placed on the mesh located in a liquid reservoir sothat the sample container 530 was raised slightly above the bottom ofthe liquid reservoir. As the mesh, those which did not affect themovement of the physiological saline solution into the sample container530 were used. During saturation, the height of the physiological salinesolution could be adjusted such that the surface within the samplecontainer was defined by the swollen super absorbent polymer, ratherthan the physiological saline solution.

At the end of this period, the assembly of the plunger 536 and weight548 was placed on the swollen super absorbent polymer 568 in the samplecontainer 530 and then the sample container 530, plunger 536, weight 548and swollen super absorbent polymer 568 were removed from the solution.Thereafter, before GBP measurement, the sample container 530, plunger536, weight 548 and swollen super absorbent polymer 568 were placed on aflat, large grid non-deformable plate of uniform thickness for about 30seconds. The height from the top of the weight 548 to the bottom of thesample container 530 was measured again by using the same thicknessgauge as previously used. Then, the height measurement value of thedevice in which the plunger 536 equipped with the weight 548 was placedin the empty sample container 530 was subtracted from the heightmeasurement value of the device including the swollen super absorbentpolymer 568, thereby obtaining the thickness or height “H” of theswollen super absorbent polymer.

For the GBP measurement, 0.9 wt % physiological saline solution wasflowed into the sample container 530 containing the swollen superabsorbent polymer 568, the plunger 536 and the weight 548. The flow rateof a physiological saline solution into the container was adjusted tocause the physiological saline solution to overflow the top of thecylinder 534, thereby resulting in a consistent head pressure equal tothe height of the sample container 530. Then, the quantity of solutionpassing through the swollen super absorbent polymer 568 versus time wasmeasured gravimetrically using the scale 602 and beaker 603. Data pointsfrom the scale 602 were collected every second for at least sixtyseconds once the overflow has started. The flow rate (Q) passing throughthe swollen super absorbent polymer 568 was determined in units ofgrams/second (g/s) by a linear least-square fit of fluid passing throughthe sample 568 (in grams) versus time (in seconds).

Using the data thus obtained, the GBP (cm²)) was calculated according tothe following Calculation Equation 3.K=[Q×H×μ]/[A×ρ×P]  (3)

in Calculation Equation 3,

K is a gel bed permeability (cm²),

Q is a flow rate (g/sec)

H is a height of swollen super absorbent polymer (cm),

μ is a liquid viscosity (poise) (about 1 cP for the test solution usedwith this Test),

A is a cross-sectional area for liquid flow (28.27 cm² for the samplecontainer used with this Test),

ρ is a liquid density (g/cm³) (about 1 g/cm³, for the physiologicalsaline solution used with this Test), and

P is a hydrostatic pressure (dynes/cm²) (normally about 7,797 dyne/cm²).

The hydrostatic pressure was calculated from P=ρ×g×h, where ρ is aliquid density (g/cm³), g is a gravitational acceleration (nominally 981cm/sec²), and h is a fluid height (for example, 7.95 cm for the GBP Testdescribed herein)

At least two samples were tested and the results were averaged todetermine the free swell GBP of the super absorbent: polymer, and theunit was converted to darcy (1 darcy=0.93692×10⁻⁸ UW).

(6) Vortex Time of Super Absorbent Polymer

The vortex time of the super absorbent polymer was measured in secondsaccording to the method described in International Publication WO1987/003208.

Specifically, the vortex time (or absorption rate) was calculated bymeasuring in seconds the amount of time required for the vortex todisappear after adding 2 grams of a super absorbent polymer to 50 mL ofphysiological saline solution and then stirring the mixture at 600 rpm.

(7) Rewetting Properties of Super Absorbent Polymer

The rewetting properties of the super absorbent polymers of Examples 1to 5 and Comparative Example 1 were evaluated by modifying a knownmeasurement method of absorbency under load.

First, a super absorbent polymer having a particle diameter of 300 to600 μm which was passed through a U.S. standard 30 mesh screen andretained on a U.S. standard 50 mesh screen was prepared from a superabsorbent polymer for evaluating the rewetting properties.

Meanwhile, the 400 mesh stainless steel screen was attached to thebottom of a plastic cylinder having an inner diameter of 25 mm. Then, W₀(g 0.16 g) of the previously prepared super absorbent polymer wasuniformly scattered on the screen under conditions of room temperatureand relative humidity of 50%, to thereby prepare a test assembly.

Then, a first filter paper having a diameter of 25 mm was laid on the PEdish having a diameter of 80 mm, and the test assembly was placedthereon. Thereafter, 4 g of 0.9 wt % physiological saline solution wasinjected around the test assembly, so that the super absorbent polymercould absorb the physiological saline solution under no load condition.When the physiological saline solution was completely absorbed by thesuper absorbent polymer, it was left for 10 minutes so that the superabsorbent polymer was swollen sufficiently.

On the other hand, as Whatman Grade No. 4 filter paper, 10 sheets offilter papers having a diameter of 30 mm or more were overlapped toprepare a second filter paper. Then, the weight W₅ (g) of the secondfilter paper was measured.

After lifting and removing the test assembly from the first filterpaper, a piston capable of uniformly applying a load of 5.1 kPa (0.7psi) onto the swollen super absorbent polymer was added. At this time,the piston was designed so that the outer diameter was slightly smallerthan 25 mm and thus it could move freely up and down without any gapwith the inner wall of the cylinder.

Then, the test assembly to which the piston was added was placed on thepreviously prepared second filter paper. After lifting and removing thetest assembly to which the piston has been added after 2 minutes, theweight W₆ (g) of the second filter paper was again measured.

Using each of the weights thus obtained, the rewetting amount (g/g) wascalculated by the Calculation Equation 4.Rewetting Amount (g/g)=[W ₆ (g)−W ₅ (g)]/W ₀ (g)  (4)

in Calculation Equation 4,

W₀ (g) is an initial weight (g) of the super absorbent polymer, W₅ (g)is an initial weight (g) of the second filter paper, W₆ (g) is a weight(g) of the second filter paper that has absorbed a liquid leaking outfrom the super absorbent polymer swelled for 2 minutes under a load (0.7psi), after the super absorbent polymers have absorbed 25 times theirweight in a physiological saline solution for a sufficient time under noload condition.

The results of the above measurement are shown in Table 1 below.

TABLE 1 Content (wt %) Secondary Non- Vortex Rewetting Porous granuleporous CRC AUL GBP time amount particle particle particle [g/g] [g/g][darcy] [sec] [g/g] Example 1 36 25 39 30.7 19.7 57 35 0.4 Example 2 2923 48 30.1 19.4 53 37 0.6 Example 3 38 24 38 30.4 19.1 75 32 0.5 Example4 34 26 40 30.8 19.5 62 40 0.7 Example 5 32 24 44 30.3 19.9 63 39 0.7Comparative 11 18 71 31.1 18.7 60 85 1.8 Example 1

Referring to Table 1, it is confirmed that the super absorbent polymersaccording to Examples of the present invention not only are excellent inthe centrifuge retention capacity and the absorbency under load but alsohave a faster vortex time and a less rewetting amount than ComparativeExample, thereby effectively preventing the rewetting phenomenon.

EXPLANATION OF SIGN

-   500: GBP measuring device-   528: Test apparatus assembly-   530: Sample container-   534: Cylinder-   534 a: Region having an outer diameter of 66 mm-   536: Plunger-   538: Shift-   540: O-ring-   544, 554, 560: hole-   548: Annular weight-   548 a: Thru-bore-   550: Plunger head-   562: Shaft hole-   564: 100 mesh stainless steel cross screen-   566: 400 mesh stainless steel cross screen-   568: Sample-   600: weir-   601: Collection device-   602: Scale-   603: Beaker-   604: Gauge pump

The invention claimed is:
 1. A super absorbent polymer comprising:porous particles in which a plurality of pores having an average porediameter of 10 μm or more are formed on the surface; secondary granuleparticles in which primary particles having an average particle diameterof 10 μm to 100 μm are aggregated; and non-porous particles in which 0to 3 pores having an average pore diameter of 5 μm or more are presenton the surface, wherein the non-porous particles are contained in anamount of 15% by weight to 75% by weight based on the total weight ofthe super absorbent polymer, and the vortex time is 20 seconds to 70seconds.
 2. The super absorbent polymer according to claim 1, whereinthe porous particles are contained in an amount of 20% by weight to 60%by weight.
 3. The super absorbent polymer according to claim 1, whereinthe secondary granule particles are contained in an amount of 10% byweight to 40% by weight.
 4. The super absorbent polymer according toclaim 1, wherein the super absorbent polymer has a centrifuge retentioncapacity (CRC) for a physiological saline solution of about 28 g/g toabout 35 g/g; an absorbency under load (AUL) at 0.9 psi for aphysiological saline solution of about 14 g/g to about 22 g/g; and afree swell gel bed permeability (GBP) for a physiological salinesolution of about 40 darcy to about 100 darcy.